The Mysterious Genetics of the Four-Leaf Clover

So you lost your job and your lifes a mess. Youre fat, youre broke, youre bad at sex. Youre looking at your dog, hoping she doesnt die next. You need to get lucky. And whats the best way to get lucky? No, its not finding a four-leafed clover on St. Patricks Day. That kind of luck doesnt exist. No, what you need to do is go impress a bunch of drunk strangers with your knowledge of four-leaf clover genetics!

Like every other trait on every other living thing, a clovers lucky fourth leaf sprouts from DNA. But the clover genome is surprisingly complicated, and finding the genes responsibleand under what conditionsfor the four-leaf mutation is a still-unsolved biological puzzle.

There are over 300 different species of clover, but the type most associated with the rare fourth lucky leaf is the widespread white clover (so named because of the fluffy, delicious-looking white blossoms). Its like having a cat with an extra claw. We know it has a genetic basis, and a mutation that happens at a slow but regular frequency, says Wayne Parrott, who studies crop genetics at the University of Georgia. His lab has come closest to finding the genetic roots of the four-leaf mutation. We know more or less where it is on the chromosome, he says. But the clover seems to have done everything possible to make its genome inscrutable.

This is partly because the plant is so widely distributed. Native on three continents, the white clovers genome tells the story of a plant that geography tried, and failed, to split into multiple species. The white clover is an allotetrapoloid. Huh? Best to work backward on that one. -Ploid means chromosome, and -tetra means four. That means the white clover has double the amount of chromosomes as humans, mangoes, pill bugs, and most other organisms. This brings us to the allo- prefix, which means that each pair of the white clovers chromosomes comes from a different species.

Crazy, I knowbut clover arent aliens. When itwas proliferating over the globe, the clover started to split into multiple species, but then theydoubled back and started breeding again. And instead of recombining into diploid chromosomes, the clover kept both pairs. Maize and sorghum had some awkward allotetraploid years when they first started splitting about 20 million years ago (both now have just two chromosomes). Pretty exciting family life for a boring ole ground cover, right?

On top of that,white clover dont have the tidiest genealogies. The plants are promiscuous outbreeders (opposite of inbreeders, sorry Team Lannister). In fact, they are pretty much incapable of breeding with themselves (as many plants do). Combined with the quadruple chromosomes, the white clovers sex life means that it is incredibly difficult to figure out which genes came from which parent. This means inheritance studieswhich figure out if a genes expression is due to nature over nurtureare all but impossible.

But Parrotts group made an admirable accomplishment. They grew 178 plants and recordedamong other leaf traits like pattern and colorany plants with at least one four-leafed stem (A single clover plant sends out many runners). In the study, four-leafed stems favored warm conditionsthere were double the amount of outdoor four-leaf clovers in the summer than winter. And in the winter, more lucky shamrocks grew in the greenhouse than outside.

This could mean that three leaves is an adaption for colder or wetter climates. Clovers are a member of the bean family, which Parrott says have been steadily losing leaves over the millennia. If you go back, the ancestors of the bean family had a lot more leaves, he says. In the case of beans and clover, you are down to three. He says that this could be accidental, though says it is noteworthy that if having more leaves were a big advantage, then youd expect to see more of them. But, he also says that the number of leaves could be entirely accidental. Not everything happens for a reason, he says.

With their experimental data in hand, Parrotts teamstarted looking at the genome. They found at least one gene that correlated closely to the expression of three leaves. These genes were similarly somewhat suppressed in the summer months, leading to more four leaf clovers. Weather is important for a lot of plant traits, says Parrott, though hes reluctant to say it all comes down to time of year. He says one of the reasons that we see more four leafed clovers when its warm is because the plant is making more leaves in those times of year. The probability is simply higher.

And clovers can grow many more than four leavesthe Guinness record is 56 leaves on a single stem. Speaking of Guinness, if you drink enough of it today* youre sure to find plenty of six-leaf clovers, at the very least.

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The Mysterious Genetics of the Four-Leaf Clover